This invention relates to an inspiratory muscle training device with variable loading.
Inspiratory muscle training devices are well known, for example from UK Patent Specification No. 2 278 545 and U.S. Pat. No. 4,854,574. These known devices each incorporate a chamber having an opening in the form of a mouthpiece for the passage of air to be inhaled and exhaled, an inlet permitting air to be inhaled to enter the chamber and to pass to the opening, a one-way exhaust valve permitting exhaled air entering through the opening to escape from the chamber, and a valve to resist the entry of air to be inhaled into the chamber, which valve is designed to open at a constant threshold pressure. Although the threshold pressure can be varied by the user from breath to breath or session to session, the known devices effectively present a preselected constant load to inspiration. That is, the load is constant in that it is independent of flow and does not vary with time or lung volume.
The mechanical characteristics of the inspiratory muscles dictates that their strength varies according to the degree to which the lungs are inflated. Consequently we have recognised the importance of a load which varies according to lung volume during inspiration.
It is therefore an object of the present invention to provide an inspiratory muscle training device which demonstrates a resistance to inspiration that varies according to lung volume.
According to the present invention there is provided an inspiratory muscle training device with variable loading, which device comprises a chamber having an opening for the passage of air to be inhaled and exhaled, an inlet permitting air to be inhaled to enter the chamber and to pass to the opening, a one-way exhaust valve permitting exhaled air entering through the opening to escape from the chamber, and means to resist the entry of air to be inhaled into the chamber, wherein the means to resist the entry of air includes means to vary the degree of resistance in dependence upon the volume of air that has passed through the inlet.
The resistance may decrease as the volume of air that has passed through the inlet increases.
The means to resist the entry of air into the chamber may comprise a valve provided in the opening, the valve being urged by biasing means to a closed position in such a manner that the pressure differential across the valve required to open the same varies in dependence on the volume of air that has passed through the valve for a given inspiratory cycle.
Means may be provided to vary the initial pressure differential required to open the valve.
The means to vary the pressure differential in dependence upon the volume of air that has passed through the valve may comprise a lever acting between the biasing means and the valve, the lever having a movable fulcrum. The fulcrum may be movable relative to the volume of air that has passed through the valve. Movement of the fulcrum may be relatively slow initially, increasing with the volume of air that has passed through the valve. The fulcrum may be movable by way of a diaphragm, the amount of movement of the diaphragm being in relation to the volume of air that has passed through the valve.
The first-mentioned valve may be mechanically linked to a further valve which passes air at a flow rate proportional to the flow rate of air through the first-mentioned valve. Air passing through the further valve may be employed directly or indirectly to move the diaphragm.
Alternatively, the means to vary the pressure differential in dependence upon the volume of air that has passed through the valve may comprise cam means. The cam means may be movable by a rotary impeller positioned in the path of air entering the chamber.
Means may be provided to vary the rate at which the pressure differential required to open the valve changes, for example by varying the proportion of air flowing through the further valve relative to the volume of air flowing through the first-mentioned valve.